Buttondown Documentation
Carrd is a service that makes it easy to build single-page sites for anything you might want.
Carrd integrates with Buttondown. You can use a Carrd site to host a signup form that lets new people subscribe to your newsletter. To learn more, read Carrd's documentation.
You'll need to refer back to Buttondown when you want to find your API key, which you can do directly from your API requests page.